A recent optical transmission system has been further enhancing the transmission speed and has put transmission at 10 Gb/s into practice. In addition, an optical transmission system at the transmission speed 40 Gb/s has been under development. Further, there has been developed an optical transmission system in which 1000-multiplexed optical signals with a bit rates of 10 Gb/s are multiplexed in use of a wavelength multiplexing technique and the resultant wavelength-multiplexed signal is collectively amplified and transmitted.
In conjunction with enhancement in transmission speed, chromatic dispersion of an optical fiber intensifies deterioration in waveform of an optical signal and is therefore one of the causes of the restriction on transmittable distances. For this reason, a dispersion compensation fiber that compensates such chromatic dispersion of an optical fiber makes long distance transmission as far as several hundreds kilometers possible. Since the transmission speed further increased to approximately 40 Gb/s causes chromatic dispersion to more largely impinges, realization of long distance transmission as far as several hundreds kilometers prefers exact compensation for chromatic dispersion of an optical fiber and concurrently cannot neglect a variation in chromatic dispersion characteristics caused from a variation in temperature of the optical fiber and a variation in chromatic dispersion caused from polarization mode dispersion.
Optical transmission at a high transmission speed, such as 40 Gb/s as above, higher than at 10 Gb/s, tightens the restriction on the tolerance of chromatic dispersion, which accompanies desire for strict conditions on a residual amount of chromatic dispersion obtained after the chromatic dispersion has been compensated for. Therefore, dispersion has to be compensated for which is caused by a variation in temperature of an optical fiber and by polarization mode dispersion in order to reduce residual chromatic dispersion while the apparatus is in operation.
Various approaches have conventionally been proposed in order to compensate chromatic dispersion. For example, FIG. 7 is a diagram illustrating a conventional technique including a dispersion compensator, in which the drawings illustrates optical receiver 100, wavelength compensator 101, optical filter 102, optical-to-electrical converter (O/E) 103, data identifying section 104 with clock regenerating means, error detector 105, error rate calculator 106, error-rate variation amount calculator 107, compensation amount calculator 108, and dispersion compensating unit 111 placed at a distance.
Optical receiver 100 receives optical signal transmitted through an optical fiber; dispersion compensator 101 compensates chromatic dispersion of the received optical signal; optical filter 102 extracts an optical signal containing a signal components; optical-to-electrical converter 103 converts the extracted optical signal into an electric signal; data identifying section 104 extracts the clock (from the electric signal), identifies the data on the basis of the extracted clock, and inputs the data into error detector 105; and error detector 105 carries out error detection or error correction and outputs the data, serving as a received signal, to a subsequent apparatus which is however not illustrated in the drawing.
Error rate calculator 106 calculates an error rate based on an error detection signal of error detector 105 and error-rate variation amount calculator 107 calculates a variation in error ratio and inputs the error ratio into compensation amount calculator 108, which calculates an amount of dispersion control and controls dispersion compensator 101. Specifically, compensation amount calculator 108 controls the amount of dispersion control to be used in dispersion compensator 101 such that the error ratio is not increased.
The optical receiver 100 is configured to search, on the basis of signal quality information, such as error ratio, calculated from an optical signal transmitted through the optical transmission line serving as a connection path for an amount of dispersion control to be used in dispersion compensator 101 and sets the searched amount of dispersion control into dispersion compensator 101 at the initial setting stage (until communication has been established between a non-illustrated optical transmitter and optical receiver 100). Described in detail, optical receiver 100 sweeps a range of an amount of dispersion control variable in dispersion compensator 101 and successively measures the error ratio associated with the amount of dispersion control, and finally sets the amount of dispersion control associated with the most preferable error ratio into dispersion compensator 101.
Here, dispersion compensator 101 is exemplified by a configuration in which optical or mechanical variation in length of a dispersion compensation fiber controls an amount of dispersion control or a configuration in which a heater that searches for the temperature by supplying electricity corresponding to an amount of dispersion compensation is provided on the basis of knowledge of a temperature variation varying an amount of chromatic dispersion. Either configuration of a dispersion compensator requires tens of milliseconds or more to respond to, for example, a single control. Accordingly, it means that sweeping a compensatable range requires, for example, time as long as tens of seconds or more.
Therefore, if an amount of dispersion control is searched for in such a method, sweeping performed in search of an amount of dispersion control by dispersion compensator 101 usually requires relatively long time, optical receiver 100 requires a certain time for searching for an amount of dispersion control in order to secure a desired signal quality.
In other words, reduction in time required for the initial setting need reduction in time required for searching for the desired amount of dispersion control to be set.
Patent reference 1 sweeps a variable range at the initial setting to calculate a bit error rate and concurrently carries out synchronization detection such as frame synchronization. The disclosed configuration enhances the speed of searching for a amount of dispersion control by setting, sweeping the width of a range in which synchronization detection cannot be carried out larger than that of the range in which synchronization detection can be carried out.
The technique disclosed in Patent Reference 2 can be listed among conventional techniques related to the present invention.
[Patent Reference 1] Japanese Patent Application Laid-Open (KOKAI) No. 2004-236097
[Patent Reference 2] Japanese Patent Application Laid-Open (KOKAI) No. 2002-208892
However, even the technique disclosed in above Patent Reference 1 requires sweeping a range at least from the lower limit to the upper limit of the variable range of the dispersion compensator. Therefore, it is difficult for the technique to satisfactorily enhance the searching speed for an amount of dispersion control.
If the compensation at even the amount of dispersion control initially set in the dispersion controller worsens the error rate or the like due to a variation in temperature of the optical fiber, polarization mode dispersion and/or other reasons, another sweeping has to be made search of an amount of dispersion control of the dispersion compensator. The sweeping which likewise requires a relatively long time needs be repetitiously carried out even during the operation, and consequently a variation in temperature of the optical fiber, polarization mode dispersion and/or other reasons make it difficult to set a stable amount of dispersion control.
Further, as described below, it is problematically difficult to optimally compensated for dispersion caused by a phenomenon peculiar to optical transmission, that is, fluctuation in level of an optical input (i.e., fluctuation in Optical signal to Noise Ratio (OSNR)) and fluctuation in characteristics of Polarization Mode dispersion (PMD).
Specifically, dispersion of an optical signal being received by an optical receiver is represented by values on the x axis and Q penalty of the same optical signal is represented by values on the y axis, a dispersion value close to the center of abscissa is associated with a lower Q penalty value (i.e., representing more preferably quality) and the Q penalty value increases (i.e., the quality deteriorates) as departing from the center of abscissa (i.e., as the dispersion value decreases or increases from the center value). Namely, the relationship between the dispersion and the Q penalty of an optical signal in this case is represented by a parabola waveform (a tolerance curve) having the bottom at the center thereof.
In this case, fluctuation in light input level on the time axis causes the characteristics of OSNR to irregularly fluctuate on the time axis. For this reason, the waveform representing the relationship between the dispersion and Q penalty value varies with time in the y-axis direction (up and down) as illustrated in, for example, FIG. 8. In addition, the characteristics of PMD possessed by the optical transmission path varies on the time axis, so that the waveform representing the relationship between the dispersion and the Q penalty of the optical signal varies with time in the x-axis direction as illustrated in, for example, FIG. 9.
The fluctuation in characteristics of PMD and that of OSNR are not associated with each other, so that the combination of both fluctuations does not substantially vary in gradient of the tolerance curve but does randomly shifts on both x and y axes as illustrated in, for example, FIGS. 10 and 11 (see tolerance curves TC1 to TC6 in FIG. 10 and tolerance curves TC11 to TC16 in FIG. 11). Here, FIG. 10 shows an example of a shift in tolerance curve of receipt of an optical signal having transmission speed of 10 Gbit/s and FIG. 11 shows an example of a shift in tolerance curve of receipt of an optical signal having transmission speed of 40 Gbit/s.
As illustrated in FIGS. 10 and 11, the optimum amount of dispersion control when the optical receiver receives an optical signal and the Q penalty values corresponding to the optimum amount that are representing the coordinates at the bottom of a tolerance curve randomly shift on the xy coordinates. Further, an optical signal at a transmission speed as high as 40 Gbit/s has a narrower width than that of a transmission speed of about 10 Gbit/s and therefore has a stricter demand for dispersion.
FIGS. 12 and 13 show a relationship between the dispersion tolerance width and Q penalty values of an optical signal respectively at transmission speeds of 10 Gbit/s and 40 Gbit/s. Trapezoids L1 and L2 in FIGS. 12 and 13 represent a coordinate region defined by a dispersion value and a Q penalty value which can establish communication even under the influence of variations in OSNR and PMD.
As illustrated in FIG. 12, in receipt of an optical signal at a transmission speed of 10 Gbit/s, the width (i.e., the dispersion tolerance width) of dispersion value that can ensure communication establishment even if the tolerance curve shifts as illustrated in above FIG. 10 has a small variation with the variation in Q penalty value represented by the ordinate.
Namely, since the variation in Q penalty from the finest value Q11 to limit value Q12 in trapezoid L1 illustrated in FIG. 12 is followed by the variation in dispersion width from the lower base length L11 of trapezoid L1 to the upper base length L12 of trapezoid L2, dispersion tolerance has a small amount of variation in accordance with a variation in Q penalty (in other words, the slope of right oblique L13 and that left oblique L14 of trapezoid L1 with respect to variation in the ordinate direction are gentle).
In contrast, as illustrated in FIG. 13, in receipt of an optical signal at a transmission speed of 40 Gbit/s, the width (i.e., the dispersion tolerance width) of dispersion value that can ensure communication establishment when the tolerance curve shifts as illustrated in above FIG. 11 has a relatively large variation with the variation in Q penalty value represented by the ordinate.
Namely, since the variation in Q penalty from the finest value Q21 to limit value Q22 in trapezoid L2 illustrated in FIG. 13 is followed by the variation in dispersion width from the lower base length L21 of trapezoid L2 to the upper base length L22 trapezoid L2, dispersion tolerance has a large amount of variation in accordance with a variation in Q penalty (in other words, the slope of right oblique L23 and left oblique L24 of trapezoid L2 with respect to the ordinate direction are sharp).
As described above, for an optical signal transmitted at the transmission speed 40 Gbit/s, even if the OSNR value is fine because a dispersion value corresponding to either end of the lower base of trapezoid L2 in FIG. 13 is applied to the optical signal, deterioration in Q penalty value caused from a variation in OSNR due to the variation in PMD or the like as illustrated by arrow A in FIG. 13 causes the Q value to deviate from the dispersion tolerance width, so that the communication may not be secured.
Here, as illustrated by arrow B in FIG. 13, the Q penalty even which deteriorates due to PMD fluctuation or the like may remain inside the dispersion tolerance. However, as another problem, on the basis of the OSNR value in a stationary state, whether or not the deterioration in Q penalty shifts the Q penalty to an error region as illustrated by arrow A or to a region in which an error does not occur as illustrated by arrow B.
In short, the problem is also that, in spite of a preferable OSNR value, an optical signal with a transmission speed of 40 Gbit/s may have errors caused not only by narrowing the dispersion tolerance width but also by sharpening the variation in dispersion tolerance according to deterioration in Q penalty value.
In other words, as increasing in bit rate from about 10 Gbit/s to about 40 Gbit/s, the setting of dispersion of optical signal that are capable of establishing communication becomes more sensitive to OSNR, PMD and the like, and therefore, dispersion compensation requires higher accuracy.
The Patent Reference 2 discloses a technique with a configuration that chirps an optical signal at an optical transmitter in advance to compensate for both chromatic dispersion and polarization dispersion at an optical receiver. However, since the technique of the Patent Reference 2 assumes that an optical signal that is to be compensated for previously undergoes a particular process of being chirped at an optical transmitter, the Patent Reference 2 does not provide a technique to rapidly set an amount of compensation for chromatic dispersion which amount minimizes the influence of a variation in temperature of an optical fiber and the influence of PMD without depending on whether or not an optical signal to be received has been chirped.